Generally the sensitivity of spectroscopic reflectometry and ellipsometry (SE) systems to certain variables of a sample under test is closely related to the angle of incidence (AOI) of the incident beam falling onto the sample. Current SE systems are configured with fixed angles of incidence. One typical SE system measures the thickness of a thin silicon oxide (SiO2, or “oxide” as it is commonly termed) film on a silicon substrate. The sensitivity to the oxide thickness is maximized when the AOI is set close to the Brewster angle of the silicon substrate. This leads to design of SE systems with an AOI at approximately 71 degrees, which has been an optimized system setup for most film measurement applications.
The second angle of the incident beam in a SE system that determines the measurement condition is the azimuthal angle. This angle, which is defined in the plane of the sample, has no impact on measuring film properties. It has an impact, however, when the sample is a feature, not a thin film. Recently SE systems are also used for measuring line profile of periodic features, and the choice of azimuthal angle has become significant. Several SE systems for measuring line profile or periodic structures are described further in U.S. Pat. No. 6,483,580 by Xu et. al, entitled “Spectroscopic Scatterometer System.”
In some SE systems, there will be measurement correlation among some critical dimension (CD) variables. Correlation refers to similarities in measurement results that are collected from two different features (or -CD's) within a measured specimen. The similarity causes difficulty in distinguishing and thereby collecting useful information regarding each feature. For example, correlation of measurement results can manifest itself between the measurements of a top and a third-oxide layer in an oxide/nitride/oxide (ONO) film stack. In another example, correlation of measurement results occurs when measuring the height of gratings or contacts and the thickness of one or more underlayers.
One current technique for optimizing an SE system's sensitivity to profile variables and reducing correlation between two CDs or film variables involves fixing one of the variables while the other variable is allowed to float to fit the measured spectral data. However, this technique does not effectively reduce correlation because one cannot know the exact value of the variables to be fixed. As a result, all the variations in these two variables are wrapped into the variable that is allowed to float, which leads to an inaccurate measurement.
Some SE systems can use varying angles of incidence, however none of these systems are applicable to critical dimension measurements. For example, one simple SE system design involves changing both the direction of an incident beam and a collection beam simultaneously. Such systems use a “2-θ” scanning scheme is based on a simple synchronized rotating element that adjusts both the incident beam and the collection beam. J. A. Wollam Co., Inc. has a series of products based on this scheme, which are termed as VASE® (Variable Angle Spectroscopic Ellipsometry).
A modified version of the “2-θ” scanning scheme is described in “Angular Scanning Mechanism for Ellipsometers,” by D. M. Byrne and D. L. MacFarlane, Applied Optics, 30(31), 4471-4473, (1991), in which two flat turning mirrors are used to change the angle of the incident beam and the reflection beam simultaneously. A significant drawback of this scheme is the requirement of two synchronized rotations. As the technology node continuously decreases, heading to 45 nm and lower, the performance requirements for metrology tools are also getting higher and higher. To meet leading-edge specifications of precision, accuracy, and tool-to-tool matching for CD measurements, it is critical to calibrate system parameters accurately and maintain a high level of stability for these parameters during measurements. The AOI is one of these critical system parameters that should be exactly calibrated and stabilized. The above VASE design, as a result of two rotating elements, is difficult to calibrate and has difficulty maintaining its AOI. This is the major hurdle preventing the above VASE design from being adopted in semiconductor production lines. As a result, the main applications of systems based on this scheme are typically found in research labs. Again, note that this technology has not been applied to CD measurements. Actually, the drawback of two synchronized mirror rotations would make this scanning scheme particularly difficult to implement for CD measurements.
Another SE system that varies its AOI uses an aperture that is placed in front of a collection mirror. The aperture moves linearly (for example, up and down) to select the desired AOI. The SE system is very simple since only linear movement is required. However, since the aperture allows only a portion of the illumination light to be collected, this system has very low power utilization efficiency.
Another method suggested in U.S. Pat. No. 5,166,752 (the '752 patent) uses a two-dimensional imaging array in combination with a dispersion element (such as a grating or a prism, for instance) to record signals. The measurement columns correspond to signals with a fixed wavelength but varying AOIs, while the measurement rows correspond to signals with a fixed AOI but varying wavelengths. So a pixel in a specific row and a specific column will theoretically pick only signals of a specific wavelength and a specific AOI. One significant problem related to this scheme is described in FIG. 1 of U.S. Pat. No. 5,596,406 (the '406 patent). Briefly, the problem stems from the finite size of the illumination and reflection beam. The problem is that a pixel in a specific row and column will not only record the signal corresponding to the wavelength and AOI specified by the column number and row number, but the signal from adjacent wavelength and AOI may also fall onto this pixel, thereby degrading both the spectral and the angular resolutions.
To overcome the above problem, '406 patent suggests the use of a rectangular aperture, which is elongated in one direction and placed in front of the dispersion element. In this way, only signals corresponding to the azimuth angle parallel to the long-direction of the aperture is allowed to be picked up, which reduces the three dimensional data cube (AOI, azimuth angle, wavelength) into a two dimensional “data plane” (AOI, wavelength). As a result, this scheme effectively eliminates the resolution degradation problem in the '752 patent. Unfortunately, it creates its own problem, namely, the light power utilization is much lower because only a small portion of light corresponding to a given azimuth angle is collected.
In view of the foregoing, there are continuing efforts to provide improved spectroscopic reflectometry and ellipsometry systems that are sensitive to certain profile variables, and which can reduce the measurement correlation between different critical dimensions.